1. Field of the Invention
The present invention relates to a multi-layer ceramic chip capacitor or other multi-layer ceramic electronic device and a method for production of the same.
2. Description of the Related Art
A multi-layer ceramic chip capacitor, one example of a multi-layer ceramic electronic device, has a capacitor device body comprised of alternately stacked dielectric layers and internal electrode layers. A pair of external electrodes conductive with the alternately stacked internal electrodes inside the capacitor device body is formed at the two ends of the capacitor device body.
In general, a solder containing lead (Pb) is used for fixing and mounting a multi-layer ceramic capacitor on a circuit board. Therefore, a tin (Sn) or Sn/Pb plating is formed on the outermost layer of the external electrodes to improve the bonding strength of the capacitor and solder wettability.
When considering the effect of lead on the environment, however, it is preferable to fix and mount the capacitor on the circuit board without using a solder containing lead (lead-free mounting). Fixing and mounting using a conductive adhesive is therefore being studied.
If a conductive adhesive is used for mounting a conventional multi-layer ceramic chip capacitor to a circuit board, however, the following inconveniences occur.
First, the Sn or Sn/Pb plating formed on the outermost layer of the external electrodes oxidizes faster in a high temperature, high humidity environment. If the plating at the outermost layer of the external electrodes oxidizes, the connection resistance (ESR) becomes higher and, in the functions of the capacitor, the dielectric loss (tanxcex4) becomes greater and inconveniences occur in the electrical circuit.
Second, with repeated heat cycles, microcracks occur between the conductive adhesive and the external electrodes. If microcracks occur between the conductive adhesive and external electrodes, the bonding strength of the capacitor and circuit board may degrade, the capacitor may stop functioning on the electrical circuit, and the capacitor may even fall off the circuit board.
An object of the present invention is to provide a multi-layer ceramic chip capacitor or other multi-layer ceramic electronic device having external electrodes not very liable to oxidize even if mounting the device on a circuit board using a conductive adhesive and resistant to formation of cracks with the conductive adhesive even if subjected to repeated heat cycles and a method for producing the same.
To achieve the above object, according to a first aspect of the present invention, there. is provided a multi-layer ceramic electronic device comprising a device body comprised of alternately arranged dielectric layers and internal electrode layers and external electrodes formed on an outer surface of the device body, outer layers of the external electrodes comprised of a conductive material mainly comprised of silver and further including palladium (corresponding to later explained third conductive material) and glass ingredient (corresponding to later explained second glass ingredient).
Preferably, each of the external electrodes have an undercoat layer directly formed on the outer surface of the device body so as to be electrically connected with at least part of the internal electrode layers, an intermediate layer formed on an outer surface of the undercoat layer, and the outer layer formed on an outer surface of the intermediate layer; the undercoat layer includes a first conductive material mainly comprised of copper and a first glass ingredient; the intermediate layer includes a second conductive material mainly comprised of a copper-palladium-based alloy; and the outer layer includes a third conductive material mainly comprised of silver and further containing palladium and a second glass ingredient.
Preferably, the second glass ingredient includes a glass ingredient of the same composition as the first glass ingredient.
Preferably, the first glass ingredient includes a strontium borosilicate-based glass.
Preferably, the internal electrode layer includes a base metal.
Preferably, the undercoat layers are formed on only two end faces of the device body.
To achieve the above object, according to a second aspect of the present invention, there is provided a method of producing a multi-layer ceramic electronic device comprising the steps of coating on an outer surface of a device body comprised of alternately arranged dielectric layers and internal electrode layers an undercoat layer paste including a first conductive material mainly comprised of copper and a first glass ingredient so as to be electrically connected with at least part of the internal electrode layers to thereby form a pre-baking undercoat layer; coating on an outer surface of said pre-baking undercoat layer an outer layer paste including a third conductive material mainly comprised of silver and further including palladium and a second glass ingredient to thereby form a pre-baking outer layer; and co-baking said pre-baking undercoat layer and said pre-baking outer layer at a temperature of over 600xc2x0 C. and less than 800xc2x0 C. to form an undercoat layer and outer layer and form between the undercoat layer and outer layer an intermediate layer including a second conductive material mainly comprised of a copper-palladium-based alloy.
Preferably, the second glass ingredient includes a glass ingredient of the same composition as the first glass ingredient.
Preferably, the undercoat layer paste is coated to form the pre-baking undercoat layer on only two end faces of the device body.
That is, the multi-layer ceramic electronic device according to the present invention has external electrodes formed on the outer surface of a device body comprised of alternately arranged dielectric layers and internal electrode layers, and the outer layer of the external electrodes is formed of a specific material.
Therefore, even if the multi-layer ceramic electronic device of the present invention is mounted on a circuit board using a conductive adhesive not containing lead (Pb) under a high temperature, high humidity environment, there is little liability of the outermost layer of the external electrodes being oxidized. Further, even with repeated heat cycles, there is resistance to formation of microcracks between the external electrodes and the conductive adhesive.
As a result, if using the multi-layer ceramic electronic device according to the present invention, it is possible to suppress the connection resistance (ESR), reduce the dielectric loss (tanxcex4), and maintain a high bonding strength to the circuit board, so it is possible to reliably prevent inconveniences on the electrical circuit, detachment from the circuit board, and other matters and to improve the reliability.
In the present invention, by giving the external electrodes an undercoat layer formed directly on the outer surface of the device body and including in the undercoat layer a first conductive material mainly comprised of copper having a good wettability with the base metal included in the internal electrode layers, it is possible to maintain a good bonding strength between the undercoat layer and the internal electrode layers.
In the present invention, by giving the external electrodes an intermediate layer formed on the outer surface of the undercoat layer and including in the intermediate layer a second conductive material mainly comprised of a silver-palladium-based alloy, it is possible to maintain a good bonding strength at the interface between the undercoat layer and a later explained outer layer.
In the present invention, by giving the external electrodes an outer layer formed on the outer surface of the intermediate layer and including in the outer layer a third conductive material mainly comprised of silver, even if mounting the device on a circuit board using a conductive adhesive not containing lead (Pb), there is little liability of the outer layer being oxidized and there is little liability of the interface between the internal electrode layers and the external electrodes being oxidized at the time of baking the external electrodes to the device body.
In the present invention, by including in the second glass ingredient contained in the outer layer a glass ingredient of the same composition as the first glass ingredient contained in the undercoat layer (preferably strontium borosilicate-based glass), it is possible to form all of the external electrodes formed on the outer surface of the device body as a dense bonded layer.
In the method of producing a multi-layer ceramic electronic device according to the present invention, a pre-baking undercoat layer and pre-baking outer layer formed on the outer surface of a device body comprised of alternately arranged dielectric layers and internal electrode layers are baked at a specific temperature. Due to this, an undercoat layer and outer layer are formed and a specific intermediate layer is created between the undercoat layer and outer layer. When mounting the multi-layer ceramic electronic device obtained in this way on a circuit board using a conductive adhesive not containing lead (Pb) under a high temperature, high humidity environment, the outer layer is not very liable to be oxidized and there is resistance to formation of microcracks between the outer layer and the conductive adhesive even with repeated heat cycles. Therefore, according to the method of the present invention, it is possible to produce a multi-layer ceramic electronic device improved in reliability. Further, in the method of the present invention, by not performing electroplating and by co-baking the pre-baking undercoat layer and pre-baking outer layer rather than performing them separately, the production process is simplified, so it is possible to produce a multi-layer ceramic electronic device improved in reliability.
In the method of the present invention, by coating an undercoat layer paste to form the pre-baking undercoat layers on only the two end faces of the device body, it is possible to suppress the increase of thickness around the peripheral side edges of the device body even if thickness of the external electrodes formed by the later baking increases, so it is possible to accurately obtain the dimensions of the external electrodes.
The multi-layer ceramic electronic device is not particularly limited, but may be for example a multi-layer ceramic chip capacitor, piezoelectric element, chip inductor, chip varistor, chip thermistor, chip resistor, other surface mounted (SMD) chip type electronic device etc.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-124132 (filed on Apr. 25, 2000), the disclosure of which is expressly incorporated herein by reference in its entirety.